In-situ assaying for uranium in rock formations

ABSTRACT

The specification discloses a technique for assaying for delayed fission neutrons from uranium to obtain a quantitative measure of uranium ore grade. In one embodiment, a pulsed neutron source of 14-Mev neutrons and a neutron detector are located in a borehole at the level of a formation of interest. The source is operated cyclically to irradiate the formations with bursts of fast neutrons, and the resulting neutrons from the formations are detected. Measurements are made of those neutrons detected between neutron bursts and indicative of delayed neutrons emitted as a result of neutron fission of uranium. Measurements also are obtained in a nonore-bearing formation to record the count of delayed neutrons emitted from oxygen when irradiated. These measurements are compared with those obtained in the ore-bearing formations of interest to correct for the effect of the oxygen background.

United States Patent Givens et al.

[15] 3,686,503 [451 Aug. 22, 1972 [54] IN-SITU ASSAYING FOR URANIUM INROCK FORMATIONS [72] Inventors: Wyatt W. Givens; Richard L. Caldwell;William R. Mills, Jr., all of Dallas, Tex.

[73] Assignee: Mobil Oil Corporation [22] Filed: May 18, 1970 [21] Appl.No.: 38,224

[52] US. Cl. ..250/83.l, 250/836 W [51] Int. Cl ..G01t 3/00 [58] Fieldof Search ..250/83.l, 83.6 S, 83.6 W;

[56] References Cited UNITED STATES PATENTS 3,456,113 8/1969 Keepin..250/83.l

Primary ExaminerJames W. Lawrence Assistant Examiner-Davis}. V i vNEUTRON GEN. TUBE a3 84 IO AMP AMP P. 5. FOR COUNTERS a ELEC- TRONICSCONTROL UNIT H.V. PULSE ION SOURCE PULSE NEUTRON SOURCE THERMAL NEUTRONDETECTOR Attorney-William J. Scherback, Frederick E. Dumoulin, Arthur F.Zobal, Andrew L. Gaboriault and Sidney A. Johnson [57] ABSTRACT Thespecification discloses a technique for assaying for delayed fissionneutrons from uranium to obtain a quantitative measure of uranium oregrade. In one embodiment, a pulsed neutron source of l4-Mev neutrons anda neutron detector are located in a borehole at the level of a formationof interest. The source is operated cyclically to irradiate theformations with bursts of fast neutrons, and the resulting neutrons fromthe formations are detected. Measurements are made of those neutronsdetected between neutron bursts and indicative of delayed neutronsemitted as a result of neutron fission of uranium. Mea-. surements alsoare obtained in a nonore-bearing formation to record the count ofdelayed neutrons emitted from oxygen when irradiated. These measurementsare compared with those obtained in the orebearing formations ofinterest to correct for the effect of the oxygen background.

. 28 8 Claims, 1 Drawing Figure POWER SOURCE no v 40o HERTZ 74 TIME BASE5o- GEN. 73 51 r DELAY a DELAY a GATE GATE GEN. GEN.

I5 GATED GATED SCALER SCALER IIIIU] Patented Aug. 22, 1972 3,686,503

47 (P PowER souRcE IIO v 400 HERTZ 74 20 TIME 46 W BASE 29 GEN. 73

I 45 DELAY a DELAY 8 AMP GATE GATE 67 GEN. GEN. M 27 M.- L I5 I GATEDGATED l l scALER SCALER Rs. FOR IIIIU] 72 511111 COUNTERS a ELEc- 49 TTRON'CS PULSE Ht PULSE HI DISC DISC 35 48 70 CONTROL UNIT H.V. PuLsEION- PULSE Ht souRcE DISC PULSE 82 Egg? T 5555 cRM NEUTRON NEUTRON GENTUBE SOURCE II THERMAL NEuTRoN DETECTOR MONITOR I4 60 WYATT W. GIVENSRICHARD L.CALDWELL WILLIAM R. MILLS, JR.

INVENTORS MQW ATTORNEY IN-SITU ASSAYING FOR URANIUM IN ROCK FORMATIONSBACKGROUND OF THE INVENTION Natural gamma-ray logging currently is usedas a qualitative indicator of uranium mineralization in an earthformation penetrated by a borehole, i.e., a high gamma-ray count rate ona natural gamma-ray log suggests a mineralized region. The naturalgamma-ray logs obtained by conventional practice cannot be used as areliable quantitative measure of uranium ore grade. This is dueprimarily to conditions of disequilibrium which exist between the parenturanium-238 and the daughter elements, especially bismuth-214, whichemit the bulk of gamma rays contributing to the natural gamma-ray log.Disequilibrium has been found to exist when the radioactive daughterelements, through various processes such as leaching, become separatedfrom the parent uranium. If sufficient time has not elapsed between theseparation of the uranium parent and the radioactive daughter elements,a relatively low natural gamma-ray activity may be present at the actualore body. On the other hand, one can find high natural radioactivitycoming from the separated daughters with little or no uranium present.

In addition to the above, other elements, particularly potassium andthorium, emit natural gamma radiation. This radiation, when detected andrecorded, reduces the effectiveness of a natural gamma-ray log as aquantitative measure of uranium ore grade.

Current exploration practice is carried out by drilling explorationholes extensively on a widely separated pattern and very extensively ona closer pattern after a good show of mineralization is found by naturalgamma-ray logging. Core holes are drilled and the cores extensivelyassayed chemically to quantitatively evaluate the ore deposit. Thispractice, however, is very expensive. For example, the costs of coring ahole and chemically assaying the cores are seven to ten times the costof an exploration hole. Moreover, the presently used technology missesmany ore bodies because all natural gamma-ray anomalies suggestive ofore grade mineralization cannot be confirmed because of the prohibitivecosts of coring and chemical assaying.

SUMMARY OF THE INVENTION In accordance with the present invention, thereis provided an inexpensive, in-situ assay method for quantitativelymeasuring uranium ore grade. In subsurface assaying operations, a toolcontaining a source of neutrons and a neutron detector are located in aborehole at the level of a formation of interest. The source is operatedcyclically to irradiate a zone in said formation with neutrons. Neutronsresulting from the irradiation of said zone in said formation aredetected and recorded to obtain a record of delayed neutrons emitted asa result of neutron fission of uranium. Recording takes place withineach cycle beginning at a time period after the source neutrons havedied away via absorption in the formation.

Since the present invention detects neutrons emitted as a result of thefission process, a measure of uranium can be obtained which isunaffected by disequilibrium. Although thorium may emit fissionneutrons, its effect normally will be small.

When a l4-Mev neutron source is employed, oxygen-l7 in the formationswill be activated. In the decay process of the induced activity, delayedneutrons are emitted, resulting in a delayed neutron background. Inaccordance with another aspect of the present invention, there isprovided a technique for reducing the effect of these neutrons on therecords obtained. The source and detector are located at the level of anonuranium-bearing formation. The source is operated cyclically toirradiate the nonuranium-bearing formation with bursts of fast neutronsspaced in time, and neutrons resulting from the irradiation of theformation are detected. A record is obtained of the neutrons emitted byoxygen as a result of irradiation for comparison with the count ofneutrons recorded at the formation of interest to compensate for theeffect of delayed neutrons from oxygen in the formation of interest.

BRIEF DESCRIPTION OF THE DRAWING The FIGURE illustrates a borehole tooland recording system for carrying out the invention.

DETAILED DESCRIPTION OF THE INVENTION Referring to the Figure, therewill be described the present invention and a system for obtaining aquantitative measure of uranium ore grade in formations of interesttraversed by a borehole illustrated at 10. The formation to beinvestigated or assayed is shown at 11. This formation is foundinitially from the relatively high count rate on a natural gamma-ray logpreviously obtained in the borehole. In order to determine whetheruranium in fact is present and to obtain a quantitative measure of theore grade, a borehole tool 12 is lowered into the borehole to thisformation. The tool 12 contains a neutron source 13 and a thermalneutron detector 14. In one embodiment, the source 13 is anaccelerator-type source which produces l4-Mev neutrons. The tool 12 ispositioned to locate the source and the detector at the level of theformation 11, and the source is operated cyclically to produce bursts offast neutrons spaced in time to irradiate the formation. In theformation matrix, many of the fast neutrons from the source aremoderated or slowed to thermal energies. Both thermal and fast neutronsreact with uranium, if present, for the production of delayed fissionneutrons. These fission neutrons are slowed in the formation to thermalenergies and are detected by the thermal neutron detector 14 whichproduces an output pulse for each neutron detected. The output of thedetector 14 is transmitted to the surface to a gated scaler 15.

Cyclic activation is carried out whereby a zone or region in theformation 11 is irradiated cyclically, and counting by the scaler 15 isdelayed following each irradiation period to obtain a measurepredominantly of the delayed fission neutrons emitted from uranium. Theprocess is repeated and the counts in the scaler 15 are accumulated.

The table following sets forth six groups of delayed fission neutronsfrom uranium. These neutrons are emitted from fission of uranium bythermal or fast neutrons. A detailed discussion of this phenomenon canbe found in NUCLEAR PHYSICS by Irving Kaplan, Addison-Wesley PublishingCompany, Inc.,

Reading, Massachusetts, Palo Alto, London, 1955, 1963, Second Edition,Chapter 19.

As can be understood from the table, the more abundant delayed neutrongroups have shorter half lives, the most abundant group having a halflife of about 2.3 seconds. In one embodiment, the source 13 and thescaler 15 are operated to emphasize the shorter-lived, delayed fissiongroups. In this respect, the source 13 may be operated to produce veryshort neutron bursts at a repetition rate within the range of from oneto five bursts per second. In one embodiment, the source may be operatedat two bursts per second, each burst having a duration of about 3microseconds. Cyclic irradiation may be carried out for a period ofabout minutes. During this time, the detector 14 may be operatedcontinuously to detect thermal neutrons. The scaler is operated to countonly thermal neutrons detected between neutron bursts and beginning at atime period after the moderated neutrons from the source have died away,i.e., have been absorbed by the formation. Complete die away of thesource neutrons in most formations takes place in the order of 2 to 3milliseconds following the termination of each neutron burst. In orderto ensure that no moderated source neutrons are counted, scaler 15 isoperated to begin counting at 5 milliseconds after each neutron burstand counts until the beginning of the next neutron burst. It then stopscounting but begins counting again within the same corresponding timeperiod during the next cycle. Counting by the scaler 115 also is carriedout for about a 5-minute period.

The numerical output of the scaler is a function of the number ofuranium atoms per unit volume of the formation which are directlyrelatable to uranium ore grade. As mentioned above, although otherelements such as thorium may produce fission neutrons, the cross sectionof these elements for the fission process is very small. Hence, theeffect of thorium is insignificant except at very high concentrations.

When a l4-Mev source is used, oxygen in the formation 11, however, willproduce a neutron background level which contributes to the neutronsdetected and counted by the scaler 15. When oxygen-l7 is irradiated withneutrons of energy greater than 7.93 Mev, the following reaction takesplace:

The resulting neutrons produced have a maximum energy of about 2.2 Mev.Since the half life of the beta decay is of the order of 4.14 seconds,these neutrons are produced within the same time period that theshorter-lived fission groups from uranium are produced. Thus, theycontribute to the count obtained by the scaler 15. In low uranium oregrades, correction should be made for the oxygen background. The oxygencontribution is corrected by counting the oxygen background in anonore-bearing formation and comparing this count with the total countobtained in the ore-bearing formation of interest. It is found thatcorrections may be obtained in this manner since there is littlevariation in the change of oxygen content from formation to formationwithin a given region. In this respect, it has been found that mostformation matrices contain about fifty percent oxygen, with the changein oxygen content between formations being about seven to eight percent.In order to obtain the best estimate of oxygen background in the orezone, a clean zone containing no neutron-fission-producing ores, andpreferably close to the ore zone, is chosen to obtain oxygen backgroundmeasurements. One such zone may be the formation illustrated at 16. Inorder to obtain the background measurements, the tool 12 is raised tolocate the source 13 and the detector 14 at the level of the formation16. The source 13 and the scaler 15 then are operated in the same manneras that described with respect to the investigation of the formation 11.The counts obtained by the scaler 15 then are observed and subtractedfrom those obtained in the investigation of the formation 11. It is tobe understood that the background measurements may be obtained prior toor after the formation of interest is investigated.

The resulting count obtained after oxygen background correction is aquantitative measure of the uranium concentration. A very low count willindicate little or no uranium, while progressively higher counts willindicate higher concentrations of uranium. The exact relationshipbetween the resulting counts produced and the uranium concentration isobtained by calibration measurements carried out with the tool 12operated in a plurality of known uranium ore grades of different uraniumconcentrations.

More details of the borehole and uphole system now will be described.The logging tool 12 comprises a steel housing which is supported by acable 20. This cable is unwound and wound from and upon a drum 21 drivenby motor 22 and connection 23 in order to lower and raise the tool 12inthe borehole. Slip rings and brushes illustrated at 24 and 25,respectively, are employed to couple the conductors of cable 20 to thevarious uphole equipment for the transmission of signals and voltages.Power from an uphole power source 26 is transmitted downhole to adownhole power supply 27 by way of conductors illustrated at 28, sliprings and brushes 24 and 25, and cable conductors illustrated at 29. Forsimplicity, connection is not shown between the downhole power supply 27and the counters and other electronics.

In one embodiment, the pulsed neutron source 13 comprises a neutrongenerator tube 34 which contains a target and an ion source (not shown).Pulsing is carried out by applying a high voltage pulse (5 kilovolts inone embodiment) to the ion source and simultaneously a negative-goingpulse kilovolts in one embodiment) to the target. A source of this typeis manufactured by Kaman Nuclear of Colorado Springs, Colorado. The ionsource pulse is generated by control unit 35 and applied to the ionsource by way of conductor 36. In addition, the control unit 35generates a negative pulse which is stepped to l25 kilovolts bytransformer 37 and applied to the target. A trigger pulse generateduphole is applied periodically to actuate the control unit 35 for theproduction of a high voltage and ion source pulses for pulsing theneutron generator tube 34. In this respect, a time-base generator 40located uphole periodically generates a trigger pulse at the desiredpulsing rate, for example, two pulses per second. This pulse is applieddownhole to the control unit 35 by way of conductors 411i, 28, 29, and42.

The thermal neutron detector 14 preferably is a helium-3 detector. Itmay be cylindrical in form and of the type described in U.S. Pat. No.3,359,443. In the alternative, there may be employed one or a pluralityof helium-3 detectors of the type shown in US. Pat. No. 3,102,198. Theoutput pulses of detector 14 are preamplified at 44, amplified by way ofamplifier 45, and transmitted to the surface by way of conductor 46. Atthe surface, pulses from conductor 46 are applied to conductor 47,amplified at 48, and applied to pulse height discriminator 49 whichbiases out the background level and applies the pulses representative ofthermal neutrons detected to the scaler 15. At the surface, triggerpulses from time-base generator 40 are applied by way of conductor 50 toa delay and gate generator 51. This generator produces a gating pulsebeginning at about 5 milliseconds following termination of a neutronburst and lasting until the next trigger pulse. This gating pulse isapplied to scaler to enable the scaler for counting for the duration ofthe gating pulse.

Located within the detector 14 and adjacent the neutron source 13 is adetector or monitor 60 for ob taining. a measure of the output from thepulsed neutron source 13. The output of an accelerator-type neutronsource may vary during its operation. Hence, it is desirable to monitorthe output to know that a constant output is produced during eachassaying period or to correct or compensate for variations in neutronoutput. The response time of a conventional fast neutron detector,however, is not fast enough to detect directly and measure accuratelythe number of neutrons produced by the source when it is being operatedto produce neutrons during a very short burst time, for example, of theorder of 3 microseconds.

In one embodiment, the monitor 60 may be a gamma-ray detector which isemployed to detect the delayed gamma rays emitted from oxygen in theformations when irradiated with fast neutrons from the 14- Mev neutronsource. The reaction is Nitrogen-l6 decays by beta emission with a halflife of 7.14 seconds. The gamma rays emitted following beta decaypredominantly are 6.14-Mev gamma rays and some 7.12-Mev gamma rays. Thenumber of these gamma rays emitted are proportional to the number offast neutrons produced by the source. Moreover, they are emitted over atime period sufficient to allow representative counting by aconventional gamma-ray detector. Thus, since the oxygen in theformations is relatively constant, one may detect the delayed gamma raysfrom oxygen to obtain a measure of the neutron output produced by thesource.

In one embodiment, the monitor 60 for obtaining an indirect measure ofthe neutron source output may comprise a low-Z, plastic scintillator 63coupled to a photomultiplier tube 64.

The output of the photomultiplier tube 64 is amplified by amplifiers 66and 67 and transmitted by conductor 68 to the surface to conductor 69.This output comprises electrical pulses having heights proportional tothe electron energy of Compton interaction by gamma rays. At thesurface, the output pulses are amplified by amplifier 70 and applied topulse height discriminator 71. This discriminator is adjusted to passonly those pulses representative of gamma radiation detected and havingenergy above 3 Mev or slightly higher. This is done to bias out thelower energy background. The output of pulse height discriminator 71 isapplied to gated scaler 72 which produces a count proportional to thenumber of neutrons produced by the source 13. Scaler 72 is enabled forcounting between neutron bursts within the same period that scaler 15 isenabled for counting. Delay and gate generator 73 periodically isactuated by the trigger pulses produced from time-base generator 40 forthe production of a gating signal periodically to enable the scaler 72to count during this time period. After the scaler 72 records apredetermined count, a control signal is applied by way of conductor 74to turn OFF the time-base generator 40. This will terminate the downholeneutron pulsing operations as well as counting by both of the scalers 15and 72. In one embodiment, the scalers may be of the type manufacturedby Canberra Industries, Model No. 1471, or Systems DevelopmentIncorporated, Model No. 5210 frequency counter.

The use of the monitor 60 to monitor the output of the neutron sourceindirectly by measuring the delayed gamma rays from oxygen is describedand claimed in a copending application filed by Richard L. Caldwell andWyatt W. Givens on the same date as the present application is filed.This copending application is entitled METHOD OF INDIRECTLY MONITORINGTHE OUTPUT OF A PULSED NEUTRON SOURCE. its US. Ser. No. is 38,226.

During assaying operations, the photomultiplier tube 64 may be gated OFFduring each period that the source 13 is producing neutrons. This willavoid gain shift due to the high intensity of neutrons and gamma raysproduced during the neutron burst time. A gating pulse may be derivedfrom the trigger pulse from conductor 42. The photomultiplier tube 64 isturned ON between neutron bursts to allow detection and measurement ofdelayed gamma rays from oxygen.

The scintillator 63 and photomultiplier tube 64 perform a dual functionof locating the formation 11 for carrying out the assaying operations.For example, the formation 11 is selected for assaying from the highcount rate shown on a natural gamma-ray log previously obtained. Thetool 12 then is lowered into the borehole 10 with the photomultipliertube 64 in operation. Uphole, the output of scintillator 63 andphotomultiplier tube 64 is applied to a continuoustrace recorder by wayof pulse height discriminator 81 and count-rate meter 82. The chart ofthe recorder 80 is driven in correlation with depth of the tool 12 inthe borehole. This is done by reel 83 and connection 84. Thus, as thetool 12 is lowered, the scintillator will detect natural gamma radiationwhich will be recorded by the recorder 80 as a continuous trace 85. Theoperator will observe the trace 85 and when there is recorded a highcount rate corresponding to that recorded by the natural gamma-ray logpreviously obtained, the operator will know that the neutron source andthe thermal neutron detector are at the level of the formation ofinterest. Lowering of the tool 12 will be terminated and assayingoperations begun. The purpose of the pulse height discriminator 81 is tobias out the background level. In this respect, the discriminator may beadjusted to pass pulses representative of gamma radiation havingenergies above 0.1 Mev.

Although an accelerator-type neutron source was disclosed for carryingout cyclic activation for assaying for uranium, it is to be understoodthat other types of sources may be used. For example, a sourcemechanically controlled may be employed to irradiate cyclically a zonein the formations adjacent the thermal neutron detector for carrying outthe assaying operations. Mechanically controlled sources for carryingout cyclic activation have an output which is more constant than that ofan accelerator-type source, thus eliminating the need for a neutronoutput monitor. if the energy of the neutrons produced by the sourceemployed is less than 7.93 Mev, delayed neutrons from oxygen will not beproduced, thus eliminating the need for correcting for delayed neutronsfrom oxygen.

What is claimed is:

1. A method of assaying for uranium in the formations traversed by aborehole and to obtain information indicative of the interfering effectof oxygen in said formations, comprising the steps of:

locating a pulsed neutron source and a thermal neutron detector at thelevel of a nonuraniumbearing formation,

said source being productive of fast neutrons having energies of about14 Mev,

Operating said source to periodically irradiate said nonuraniumbearingformation with bursts of fast neutrons spaced in time,

detecting neutrons resulting from the irradiation of saidnonuranium-bearing formation and indicative of delayed neutrons emittedby oxygen as a result of irradiation by neutrons,

locating said source and detector at the level of a formation ofinterest suspected of containing uranium,

operating said source to periodically irradiate said formation ofinterest with bursts of fast neutrons spaced in time, and

detecting neutrons resulting from the irradiation of said formation ofinterest to detect for delayed neutrons emitted as a result of neutronfission of uranium.

2. The method of claim 1 comprising the step of:

recording the quantity of neutrons detected, at each of said formations,between neutron bursts within a time period when neutrons from saidsource have disappeared but while delayed neutrons from oxygen anddelayed fission neutrons from uranium, respectively, may be emitted.

3. A method of assaying for uranium in the formations traversed by aborehole, comprising the steps of:

locating a pulsed neutron source and a neutron detector at the level ofa formation of interest suspected of containing uranium,

operating said source to periodically irradiate said formation ofinterest with bursts of fast neutrons spaced in time,

the time between each neutron burst being sufficient to allow neutronsfrom said source to disappear but being long enough to allow delayedneutrons emitted as a result of neutron fission of uranium to appear atsaid detector,

detecting neutrons with said detector as a result of the irradiation ofsaid formations with bursts of fast neutrons, and

obtaining measurements of the quantity of neutrons detected betweenneutron bursts at a time period when neutrons from said source havedisappeared but while delayed fission neutrons from uranium may beemitted.

4. The method of claim 3 comprising the steps of:

locating said source and detector at the level of a nonuranium-bearingformation,

operating said source to periodically irradiate said nonuranium-bearingformation with bursts of fast neutrons spaced in time, and

detecting neutrons resulting from the irradiation of saidnonuranium-bearing formation and indicative of neutrons emitted byoxygen as a result of irradiation by fast neutrons for comparison withsaid neutrons detected and recorded upon irradiation of said formationof interest.

5. A method of assaying for uranium in formations traversed by aborehole, comprising the steps of:

locating a borehole tool containing a source of neutrons and a neutrondetector at the level of a formation of interest suspected of containinguranium,

operating said source cyclically to irradiate with neutrons a zone ofsaid formation of interest, detecting neutrons resulting from theirradiation of said zone of said formation of interest, and recordingthe counts of neutrons detected within each cycle of operation whendelayed neutrons resulting from neutron fission of uranium are expectedto be detected by said detector.

6. A method of locating and assaying for uranium in the formationstraversed by a borehole, comprising the steps of:

obtaining a natural gamma-ray log of the formations traversed by saidborehole and, from said natural gamma-ray log, identifying formations ofinterest suspected of containing uranium,

locating a borehole tool containing a source of neutrons and a thermalneutron detector at the level of a formation of interest suspected ofcontaining uranium, operating said source cyclically to irradiate withneutrons a zone of said formation of interest,

detecting thermal neutrons resulting from the irradiation of said zoneof said formation of interest, and

recording the counts of thermal neutrons detected within each cycle ofoperation when delayed neutrons resulting from neutron fission ofuranium are expected to be detected by said detector.

7. The method of claim 6 comprising the steps of:

locating said borehole tool at the level of a nonuranium-bearingformation,

operating said source to periodically irradiate said nonuranium-bearingformation with bursts of fast neutrons spaced in time, and

detecting neutrons resulting from the irradiation of saidnonuranium-bearing formation and indicative of neutrons emitted byoxygen as a result of irradiation by fast neutrons for comparison withsaid neutrons detected and recorded upon irradiation of said formationof interest.

of irradiation by neutrons,

locating said source and detector adjacent a rock formation of interestsuspected of containing uranium,

operating said source to periodically irradiate said formation ofinterest with bursts of fast neutrons spaced in time, and

detecting neutrons resulting from the irradiation of said formation ofinterest to detect for delayed neutrons emitted as a result of neutronfission of uranium.

72 3 UNITED STATES PATENT OFFICE CERTIFICATE OF COREC'HON Patent No. 3686 ,503 Dat d August 22 1972 Inventor(s) Wyatt w. Givens, Richard L.Caldwell, William R. Mill It is certified that error appears in theabove-identified patent and that said Letters Patent are herebycorrectedas shown below:

Under "References Cited", the issue date shown as "8/1969" should read--7/l969--.

Column 3, between lines 2 and 3, the following table was omittec "TABLEREPRESENTATIVE DELAYED FISSION NEUTRON GROUPS FROM URANIUM Grou GroupHalf Life Relative Group Seconds Abundance Column 3, line 48, that portin of reaction (1) adi'ng "0 (n,p)N should read --O (n,p)N

Signed and sealed this 9th day of January 1973.

(S EAL) Attest I EDWARD M. FLETCHER ,JR ROBERT GOTTSCHALK AttestingOfficer Commissioner of Patents mg UNITED STATES PATENT OFFICE-CERTIFICATE OF CORRECTION PatentNo. 3,686,503 Da ed August 22, 1972Inventor(s) Wyatt W. Givens, Richard L. Caldwell, William R. Mills,Jr.

It is certified that error appears in the above-identified patent andthat said Letters Patent are hereby corrected as shown below:

Under "References Cited", the issue date shown as "8/1969" should read--7/l969-.

Column 3, between lines 2 and 3, the following table was omitted:

"TABLE REPRESENTATIVE DELAYED FISSION NEUTRON GROUPS FROM URANIUM GrouGroup Half Life Relative Group Seconds Abundance l 55.72. 0.033 2 22.720.219 3 6.22 0. 196 4 2.30 0.395 5 0.61 O. 115 6 0.23 0.042" Column 3,line 48 th t porti n of reaction (1) ading "O (n,p)N should read --O(n,p)N

Signed and sealed this 9th day of January 1973.

s mm Attest:

EDWARD M.FLETCHER,JR. ROBERT GOTTSCHALK Attesting Officer Commissionerof Patents

1. A method of assaying for uranium in the formations traversed by aborehole and to obtain information indicative of the interfering effectof oxygen in said formations, comprising the steps of: locating a pulsedneutron source and a thermal neutron detector at the level of anonuranium-bearing formation, said source being productive of fastneutrons having energies of about 14 Mev, Operating said source toperiodically irradiate said nonuraniumbearing formation with bursts offast neutrons spaced in time, detecting neutrons resulting from theirradiation of said nonuranium-bearing formation and indicative ofdelayed neutrons emitted by oxygen as a result of irradiation byneutrons, locating said source and detector at the level of a formationof interest suspected of containing uranium, operating said source toperiodically irradiate said formation of interest with bursts of fastneutrons spaced in time, and detecting neutrons resulting from theirradiation of said formation of interest to detect for delayed neutronsemitted as a result of neutron fission of uranium.
 2. The method ofclaim 1 comprising the step of: recording the quantity of neutronsdetected, at each of said formations, between neutron bursts within atime period when neutrons from said source have disappeared but whiledelayed neutrons from oxygen and delayed fission neutrons from uranium,respectively, may be emitted.
 3. A method of assaying for uranium in theformations traversed by a borehole, comprising the steps of: locating apulsed neutron source and a neutron detector at the level of a formationof interest suspected of containing uraNium, operating said source toperiodically irradiate said formation of interest with bursts of fastneutrons spaced in time, the time between each neutron burst beingsufficient to allow neutrons from said source to disappear but beinglong enough to allow delayed neutrons emitted as a result of neutronfission of uranium to appear at said detector, detecting neutrons withsaid detector as a result of the irradiation of said formations withbursts of fast neutrons, and obtaining measurements of the quantity ofneutrons detected between neutron bursts at a time period when neutronsfrom said source have disappeared but while delayed fission neutronsfrom uranium may be emitted.
 4. The method of claim 3 comprising thesteps of: locating said source and detector at the level of anonuranium-bearing formation, operating said source to periodicallyirradiate said nonuranium-bearing formation with bursts of fast neutronsspaced in time, and detecting neutrons resulting from the irradiation ofsaid nonuranium-bearing formation and indicative of neutrons emitted byoxygen as a result of irradiation by fast neutrons for comparison withsaid neutrons detected and recorded upon irradiation of said formationof interest.
 5. A method of assaying for uranium in formations traversedby a borehole, comprising the steps of: locating a borehole toolcontaining a source of neutrons and a neutron detector at the level of aformation of interest suspected of containing uranium, operating saidsource cyclically to irradiate with neutrons a zone of said formation ofinterest, detecting neutrons resulting from the irradiation of said zoneof said formation of interest, and recording the counts of neutronsdetected within each cycle of operation when delayed neutrons resultingfrom neutron fission of uranium are expected to be detected by saiddetector.
 6. A method of locating and assaying for uranium in theformations traversed by a borehole, comprising the steps of: obtaining anatural gamma-ray log of the formations traversed by said borehole and,from said natural gamma-ray log, identifying formations of interestsuspected of containing uranium, locating a borehole tool containing asource of neutrons and a thermal neutron detector at the level of aformation of interest suspected of containing uranium, operating saidsource cyclically to irradiate with neutrons a zone of said formation ofinterest, detecting thermal neutrons resulting from the irradiation ofsaid zone of said formation of interest, and recording the counts ofthermal neutrons detected within each cycle of operation when delayedneutrons resulting from neutron fission of uranium are expected to bedetected by said detector.
 7. The method of claim 6 comprising the stepsof: locating said borehole tool at the level of a nonuranium-bearingformation, operating said source to periodically irradiate saidnonuranium-bearing formation with bursts of fast neutrons spaced intime, and detecting neutrons resulting from the irradiation of saidnonuranium-bearing formation and indicative of neutrons emitted byoxygen as a result of irradiation by fast neutrons for comparison withsaid neutrons detected and recorded upon irradiation of said formationof interest.
 8. A method of assaying for natural uranium-bearing rock,comprising the steps of: locating a pulsed neutron source productive offast neutrons having energies greater than 7.93 Mev and a thermalneutron detector adjacent a nonuranium-bearing rock formation, operatingsaid source to periodically irradiate said nonuranium-bearing formationwith bursts of fast neutrons spaced in time, detecting neutronsresulting from the irradiation of said nonuranium-bearing formation andindicative of delayed neutrons emitted by oxygen as a result ofirradiation by neutrons, locating said source and detector adjacent arock formation of interest suspected of containing uranium, operatingsaid source to periodically irradiate said formation of interest withbursts of fast neutrons spaced in time, and detecting neutrons resultingfrom the irradiation of said formation of interest to detect for delayedneutrons emitted as a result of neutron fission of uranium.